Energy dependent changes in the cytochromes of the mitochondrial respiratory chain

Abstract

In intact mitochondria a stoichiometric coupling exists between cytochrome a 3 and the hydrolysis of adenosine triphosphate (ATP). In each case the modification of one cytochrome a 3 (measured as a spectral change) is coupled to the hydrolysis of one ATP molecule. When both cytochromes a 3 and a are reduced the measured equilibrium constant is 0.06 m −1 but this constant is 10 3 M −1 when both cytochromes are oxidized. When the sixth ligand for cytochrome a 3 is an externally added ligand (HCN, H 2S, CO, NO) the equilibrium constant is different for each ligand, suggesting that the ATP induced modification is of the fifth ligand but that it is energetically dependent on the chemical nature of the sixth ligand. The measured half-reduction potentials for cytochromes a 3 and b T are dependent on the concentrations of added ATP, adenosine diphosphate (ADP), and orthophosphate. The relationship is consistent with a ligand exchange mechanism in which the ligand on the cytochrome is dependent on the phosphate potential ( ATP ADP × P i). The equilibrium constants obtained by the ligand exchange treatment of the E m values for cytochrome a 3 are consistent with those obtained by direct measurement of the equilibrium constants for the spectrally measured changes.